pinctrl: st: Convert to immutable irq_chip

Convert the driver to immutable irq-chip with a bit of
intuition.

I switched to using irqd_to_hwirq() consistently while we
are at it.

This driver does not use the GPIOCHIP_IRQ_RESOURCE_HELPERS
as it defines its own resource reservations, simply in
order to turn IRQ lines into inputs on initialization.

Also switched the open coded calls to gpiochip_lock_as_irq()
to gpiochip_reqres_irq() so we also get the right module
reference counting.

Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230403-immutable-irqchips-v1-7-503788a7f6e6@linaro.org